1. Field of the Invention
This invention relates to an improved solid-state relay or regulator having no moving parts and, more particularly, to an improved circuit and opto-electronic semiconductor device which provides electrical isolation but optical coupling between an input current and an output current conducting path, wherein the impedance of the output path varies in response to the input current.
2. Description of the Prior Art
There is an increasing need for solid-state devices without moving parts which can replace electro-mechanical relays and regulators which regulate or switch current flow in a power circuit in response to a mechanically coupled but electrically isolated control current. As used herein, the word relay includes devices having a limited number of impedance states, e.g., either off or on, high or low, etc., as well as devices which have a continuously variable impedance, e.g. regulators. Electro-optical devices when mated with photosensitive diodes or transistors can perform this function as a solid state relay or regulator. In this case, the input and output circuits are optically rather than mechanically coupled, and there are no moving parts. Such a device has been described in U.S. Pat. No. 4,227,098 in which a light emitting diode (LED) is coupled to an MOS field effect transistor (MOSFET) by means of a series string of photovoltaic diodes. Energizing the LED causes the impedance between the source-drain terminals of the MOSFET to vary over a wide range, turning on or off, much as does a mechanical relay.
An alternative embodiment is described in U.S. Pat. No. 4,166,224 wherein an LED and photovoltaic diode string combination is used to energize a thyristor so that it can be switched into the on-state.
A significant deficiency of the prior art devices is their relatively slow switching speed compared to that normally obtainable from semiconductor devices. For example, the device of U.S. Pat. No. 4,227,098 is described as having turn-on and turn-off times of approximately one millisecond.
Accordingly, it is an object of this invention to provide an improved optically coupled solid-state relay in which the switching time, especially the turn-off time is reduced.
It is further an object of this invention to provide an improved optically coupled solid-state relay in which the switching time, especially the turn-off time is reduced without deleterious effect on the output voltage of the photovoltaic diode string coupling the LED to the output transistor.
It is an additional object of this invention to provide a discharge resistance of varying value; having a high value when light is being absorbed, and a low value when light is not being absorbed.
It is a further object of this invention to provide a variable discharge resistance in the form of an active pull-down device which is driven by a second photovoltaic diode string optically coupled to one or more input light sources such as LED's.
It is yet an additional object of this invention to provide a solid-state relay in which two input circuits exist, one of which, when energized, acts to turn the relay on and the second of which, when energized, acts to over-ride the influence of the first circuit and turn the solid state relay off or place it in an intermediate impedance state.